1. Field of the Invention
The present invention relates generally to rotational molding processes and apparatus. More particularly, the present invention relates to an improved thermally insulated drop box for holding polymer material during rotational molding.
2. Related Art
In rotational molding, a mold is simultaneously rotated about multiple (usually two) axes while being heated in an oven. Polymer material (usually powder or pellets) in the mold melts and is spread around the inside of the mold by the rotational motion. After sufficient time, the mold is removed from the oven and allowed to cool while still rotating. After sufficient cooling, rotation can be stopped and the article can be removed from the mold.
It is sometimes desirable to inject polymer material into the mold while it is rotating in the oven. To do this, a drop box or canister can be attached to the mold. The drop box is a thermally insulated container that holds the additional polymer material (again, usually powder or pellets) and shields it from melting temperatures until an appropriate time during the process. Then, a plunger or other device withdraws from the opening of the drop box to allow the additional polymer material to flow into the mold while the mold is rotating.
An important aspect of drop boxes is their thermal insulating ability. A typical drop box comprises a double-walled metal container (having, e.g. a partially conical shape) with insulation disposed between the walls. Unfortunately, the configuration of prior drop boxes tends to conduct heat to the inside of the box. This conduction is generally not sufficient to hinder one use of the drop box or subsequent uses that are temporally spaced. That is, the rate of conduction usually is not so fast as to prevent the drop box from insulating its contents while the mold and other elements heat up within the oven during rotational molding. However, it can become a problem where a drop box is very rapidly prepared for reuse, and has insufficient opportunity to cool down between uses.